Add A20 support to u-boot v2

This is a cleaned up version of luke's A20 patch set. Some patches where
reverted some cleaned up. No code changes where done.

The 915 MHz default clock patch is also reverted as we don't even know
if that is needed at all. If so, it could be an idea to have All start
at 915 MHz for concistency's sense.

This patch chould be 'squashable' over the earlier lkcl set

Signed-off-by: Oliver Schinagl <oliver@schinagl.nl>